Developers used in electrophotographic systems such as copying machine include two-component developer, which comprises carrier and toner, and single component developer, which contains no carrier. Conventional toners used in both developers are fine particles of binder resin in which colorants, such as pigment and dye, are dispersed.
One of the important properties of the toners is triboelectric chargeability. Properties required for toners include that they can generate a suitable amount of positive or negative electrostatic charge upon contacting with the carrier or charge providing device, and the amount of the charge is stable during continuous use or under an adverse environment. Binder resin, the main component of the toner may be triboelectrically chargeable but the amount of the generated electrostatic charge is not sufficient, and therefore image fogging may occur to make the resulting image unclear. In order to impart further triboelectric charging property, toners in general are added with a charge control agent so that the electrostatic charge is controlled suitably.
In these days, devices for electrophotographic process such as copying machine with organic photo conductor (OPC) are increasing. Such devices meet the demands for downsizing and lower prices. The organic photo conductors include positively charged type and negatively charged type, and the positively charged type is especially increasing because the amount of ozone generated upon charging is relatively small.
In addition, with regard to recent digital electrophotographic machines or laser beam printers, the reversal development system wherein images are developed using toners homopolar to the photo conductor becomes predominate. When the reversal development system using positively charged type organic photo conductor is employed, it is necessary to use the positively electrifiable toner.
As positive charge control agents used for positive electrophotographic toners, nigrosin dyes and quaternary ammonium bases and the likes are known and have been in practical use.
Problem to be Solved by Invention
However, there are some drawbacks that nigrosin dyes are black and therefore not suitable for color toner and that quaternary ammonium salts themselves are expensive and give low charging level. As such, positively electrifiable charge control agents that do not affect the colors of toners and give satisfactory charging level have been desired.
To solve the above-described problems, an object of the present invention is to provide a novel naphthol derivative which is suitably used as charge control agent for a positively electrifiable electrophotographic toner and the like.
Another object of the present invention is to provide a positive charge control agent having a good chargeability, an excellent dispersibility and compatibility with binder resins as well as being safe for human bodies and environments, and then be able to be applied for color toners.
Further object of the present invention is to provide a positive electrophotographic toner which can be charged uniformly within a short time period and shows a stable charging level over time. Said toner provides high quality images with high image density and less fogging.
Means for Solving the Problem
The present inventors have successfully synthesized a novel naphthol derivative starting from 2-hydroxynaphthalene-6-carboxylic acid and evaluated its properties, and then found that it has good positive electrifiablity and is suitable for a charge control agent of electrophotographic toner, and have completed the invention.
Accordingly, the present invention provides a novel naphthol derivative represented by formula (I):
wherein
R1 is selected from the group consisting of an optionally branched and optionally substituted aliphatic C1 to C20 hydrocarbon group which may have an unsaturated bond, an optionally substituted aromatic group, an optionally substituted condensed polycyclic hydrocarbon group and an optionally substituted heterocyclic group having conjugated double bonds;
R2 is selected from the group consisting of hydrogen atom, alkaline metal, an optionally branched, optionally substituted C1 to C6 alkyl and acyl group and phenylalkyl group;
Q is selected from the group consisting of an optionally branched C1 to C6 alkyl and alkoxy group, halogen atom, nitro group, nitroso group, amino group and sulfo group; and
m is an integer of 0 to 3.
The novel naphthol derivative of the present invention is suitably used as a charge control agent applied for electrophotographic toner and the like. Thus the present invention further provides a charge control agent comprising the naphthol derivative as well as an electrophotographic toner comprising the charge control agent.